1. Field of the Invention
The present invention relates generally to decoding of compressed video signals, and more specifically to a method and apparatus for decoding video signals such as MPEG-2 video in which video signals are coded according to a compression technique that supports field estimation motion compensation. The present invention is particularly suitable for applications where resolution is reduced for relieving the burden on processing signals.
2. Description of the Related Art
Need often exists for displaying MPEG-2 video signals on a device whose resolution is different from the resolution of the signal. Displaying high-definition television signals on a standard television receiver is a case in point. The usual practice is to first decode the signal and reduce its resolution in a compression process when the signal is displayed. Since fine details (high frequency components) are lost, the use of pre-processing technique is employed to remove high frequency components prior to the decoding process.
Another approach is disclosed in a paper “A Drift Free Scalable Decoder”, Masahiro Iwasaki, et al., Technical Report of IEICE (Institute of Electronics, Information and Communication Engineers), CS 94-186, DSP94-108, 1995-01. In this approach, an inverse 4×4 DCT (discrete cosine transform) process is performed on a variable-length decoded, inverse quantized signal to “downscale” its resolution. With this process, the lower frequency components of the signal are exclusively used, with a resultant decrease in the resolution of the signal. The decoded signal then undergoes a motion vector compensation process, which is followed by a process in which motion compensation is performed by halving the motion vectors. However, since the information contained in one of the fields is lost, it is impossible to meet the requirements specified by the MPEG-2 standard only with the use of the field estimation technique.
In order to solve this problem, Japanese Patent Publication P2000-59793A discloses a video decoding technique for both progressive and interlaced video signals. The known technique uses a coding scheme known as the frame DCT mode in which the video signal is encoded by treating its top and bottom fields as a single unit. During decoding, the compressed video is first subjected to an inverse frame-DCT process and then the top and bottom fields are separated from each other. The separated fields are individually subjected to an inverse field-DCT process in which low frequency components are exclusively used to downscale the resolution of the signal. Although much of the field information can be retained, the prior art requires a substantial amount of computations, which would impose a great burden on software-driven systems such as personal computers.